Isotopic Exchange in Porous and Dense Magnesium Borohydride

Abstract

Magnesium borohydride (Mg(BH 4 ) 2 )i so ne of the most promising complex hydrides presently studied for energyrelated applications.M any of its properties depend on the stability of the BH 4 À anion. The BH 4 À stability was investigated with respect to H!Dexchange.Insitu Raman measurements on high-surface-area porous Mg(BH 4 ) 2 in 0.3 MPaD 2 have shown that the isotopic exchange at appreciable rates occurs already at 373 K. This is the lowest exchange temperature observed in stable borohydrides.G as-solid isotopic exchange follows the BH 4 À + DC!BH 3 D À + HC mechanism at least at the initial reaction steps.E xsitu deuteration of porous Mg(BH 4 ) 2 and its dense-phase polymorph indicates that the intrinsic porosity of the hydride is the key behind the high isotopic exchange rates.Itimplies that the solid-state H(D) diffusion is considerably slower than the gas-solid H!De xchange reaction at the surface and it is ar ate-limiting steps for hydrogen desorption and absorption in Mg(BH 4 ) 2.Metal borohydrides (MBHs) are complex hydrides containing hydrogen-rich molecular BH 4 À anions counterbalanced by metal cations.A lkali or alkaline-earth MBHs (but Be) are ionic stable compounds,d ecomposing above 473 K with release of mostly H 2 . [1] Their stability is related to the strong BÀHbonds in BH 4 À ,which are only slightly perturbed by weak interactions with the cations. [2] Owing to the high hydrogen density,M BHs have been extensively studied for solid-state hydrogen-storage applications. [1] Thec ompounds with the highest hydrogen content, lowest stability,and those releasing mostly H 2 are most attractive.Mg(BH 4 ) 2 has one of the highest hydrogen capacities (14.8 wt %H ), [3] and DFT calculations predicted an exceptional, compared to other stable MBHs,t hermodynamics with decomposition and H 2[*] Dr.

Effect of additives, ball milling and isotopic exchange in porous magnesium borohydride

“…The isotopic exchange seems to start at 81 C as a new peak appears at 1718 cm À1 and corresponding to the reported region of symmetric B-D stretching (n B-D ). 43 This characteristic peak for B-D stretching is observed at a lower temperature than that reported by Zavorotynska et al 31 As previously reported, 31 this observation is supported by a decrease in 2v 4 and red shi in v 1 . The latter observation can be explained by the fact that the peak at $2318 cm À1 actually consists of several overlapping peaks, and is dominated by n 1 in pure Mg(BH 4 ) 2 .…”

“…The observed B-D modes are in good agreement with literature. 31,32 S1-D and S2-D have gained the most in intensity in B-D stretching followed by S4-D and S3-D, when comparing the B-H stretching mode n 3 and combined B-D stretching modes n B-D aer H / D exchange. These observations give a rst indication on the extent of exchange aer 23 h and imply that the pure materials may have higher exchange rates than those with additives.…”

“…Isotopic experiments in group I and II metal borohydrides have been carried out by different research groups. 30 Recently, Zavorotynska et al 31 demonstrated the H → D isotopic exchange reaction in porous γ-Mg(BH 4 ) 2 at much milder conditions than reported for isotopic exchange in the denser α-Mg(BH 4 ) 2 phase. 32 In α-Mg(BH 4 ) 2 , the isotopic exchange reaction was observed at 132 °C after prolonged treatment at 42 bar D 2 , whereas the exchange in porous γ-Mg(BH 4 ) 2 was recorded after ∼17 min at 100 °C under only 3 bar D 2 .…”